1. Field of the Invention
The present invention relates to a detecting apparatus for focus in a camera, and is particularly concerned with a technique for detecting focus in high precision in consideration of aberration of a photographing lens.
2. Related Background Art
Generally, an F-number of the focus detection optical system is constant regardless of a photographing lens, and a focus detecting plane is set at the optimum image plane in the F-number of the focus detection system. On the other hand, it is known that a position of the optimum image plane shifts according as a stop of the lens changes in size due to a spherical aberration of the photographing lens. Accordingly, if a stop vaue to be controlled practically at the time of photographing varies from the F-number of the focus detection system, then there may be a case where the optimum image plane is dislocated against the optimum image plane position decided by the focus detection system.
To solve the aforementioned problem, U.S. Pat. No. 4,473,287 discloses a technique for providing a means for reading a signal to identify a lens type and deciding a data on a spherical aberration of the lens which is provided in a photographing lens assembly and a means for correcting the optimum image plane position from the spherical aberration data within a camera body.
However, the prior art requires a memory coordinating a correction rate of the optimum image plane position from a type of the aberration and the F-number controlled at the time of photographing provided within the camera body. Therefore many memories will be required for covering the type of each aberration of a variety of photographing lenses. Particularly in the case of a zoom lens, since the type of aberration is totally different at every zooming position, an enormous storage is required in the camera body, creating problems of cost and capacity as well.
Then, a further problem occurs when a type of aberration of a photographing lens manufactured later does not belong to any previous type, so that there is no coordination with the camera body having such memories.
Japanese Pat. No. 208514/1984 laid open on Nov. 26, 1984 discloses a camera automatic focus detecting apparatus including a photographing lens provided with a means for storing a correction rate of the optimum image plane position coordinating with a spherical aberration characteristic.
In taking note particularly of the phenomenon that a direction of correction for matching the optimum image plane position of a photographing lens detected by a photosensor with a film plane varies according to a spherical aberration characteristic of each lens, the aforementioned prior art is characterized in that information on the correction value and direction is transferred to a focusing device of the camera body from the photographing lens, and focusing is effected so as to match the optimum image plane with the film plane with the corrections taken into consideration by the focusing device.
Contrary to the aforementioned U.S. Patent, the camera body according to the Japanese Patent is advantageous in respect of a capability to cope with every type of aberration of the photographing lens developed later, but a defect is that a correction rate incorporated on the photographing lens side is determined on the premise that the focus detection system having a specific F-number is incorporated in the camera body. Therefore if the F-number of a focus detection system of the camera body developed later varies from the previous one, then the correction rate incorporated in the lens can no longer be applied to the focus detection system.
Further, the case where the focus detection system of the camera body has a plural or variable F-number is also not accommodated.